In recent years, attention has been directed to hydrogen absorbing alloys for the negative electrodes of alkali batteries and as hydrogen storage materials. Such hydrogen absorbing alloys are prepared by alloying a mixture of a rare-earth metal and a transition metal in a specified ratio, and typical of these alloys are, for example, LaNi.sub.5, MmNi.sub.2 Co.sub.3 (wherein Mm is a misch metal), etc.
In designing various devices, it is important to measure the hydrogen absorbing/desorbing characteristics of hydrogen absorbing alloys. These characteristics are used for setting the amount of hydrogen to be retained, pressure resistance, etc., are data indispensable to the provision of safety devices and are utilized also for improving the alloy when required.
The hydrogen absorbing/desorbing characteristics are represented basically by a pressure-composition isotherm (PCT) diagram. As shown in FIG. 21, the diagram represents the relationship between the pressure (ordinate) and the amount of hydrogen absorption (abscissa) under a specified temperature condition.
FIG. 20 shows a Sieverts' device conventionally used for measuring the data required for preparing a PCT diagram.
The Sieverts' device has a sample container 161 packed with a sample 162 of the hydrogen absorbing alloy to be checked for characteristics, and a gas storage container 164 connected to the container 161 via a manual valve 163. A hydrogen supply source 166 is connected to the gas storage container 164 by a hydrogen supply pipe provided with a manual valve 165. The gas storage container 164 is connected to a hydrogen discharge pipe via a manual valve 167 and further to a vacuum pump 168. With the valves 165 and 167 closed, the pressure and temperature of hydrogen gas are measured respectively by a pressure sensor 169 and a temperature sensor which is provided on the storage container 164.
The hydrogen absorbing characteristics are measured by this device by the following procedure.
First, hydrogen gas of specified pressure is supplied from the gas storage container 164 to the sample container 161 at a specified temperature. At this time, the hydrogen content of the sample alloy is measured to calculate the amount of absorbed hydrogen (wt. % or atomic ratio). The amount of absorbed hydrogen can be calculated, for example, from the pressure difference resulting from the introduction of hydrogen and the internal volume of the system using an equation of state of the gas. The amount of absorbed hydrogen and the hydrogen gas pressure value thus obtained are plotted on a PCT diagram (o mark in FIG. 21).
Next, hydrogen gas of slightly increased pressure is supplied to the sample container, similarly followed by measurement of the hydrogen content of the sample and calculation of the amount of absorbed hydrogen. The value of hydrogen gas pressure and the amount of absorbed hydrogen are plotted on the PCT diagram.
In this way, the pressure of the hydrogen gas to be supplied is increased stepwise, the hydrogen content of the sample alloy is measured every time, and the pressure value and the amount of absorbed hydrogen are plotted. For the determination of hydrogen desorbing characteristics, the pressure is conversely decreased stepwise.
For preparing the PCT diagram from the measurement data, the items of data necessary for the alloy sample are about 20 points in the direction of absorption (pressure increase direction) and about 20 points in the direction of desorption (pressure decrease direction), i.e., about 40 points in total. When the PCT diagram is prepared at each of different specified temperatures, 40 points multiplied by the number of different temperatures is the total number of measurements.
The hydrogen gas pressure is set by adjusting the degree of opening of the manual valve 165. When the hydrogen gas pressure is to be set at a value of about 0.01 to about 0.5 MPa, it is usually necessary to vary the pressure within the range of about 0.01 to about 0.1 MPa. However, the pressure of hydrogen gas cylinders serving as the hydrogen supply source is 12 to 15 MPa when the cylinder is filled up, and the pressure is as high as about 5 to about 6 MPa even when reduced by a regulator. It is therefore difficult to finely adjust the amount of hydrogen gas to be set.
Needle valves are generally used as the valves of the device. These valves wear if opened and closed every time the measurement is made, whereas since hydrogen molecules are extremely small, the wear, if slight, permits a leak and is objectionable.
FIG. 22 shows a device proposed to reduce the opening and closing frequency of the manual valve 165 and effect automated measurement. This device has a resistance pipe 170 and an inlet valve 171 of the two-position change-over type between the manual valve 165 and the gas storage container 164. An outlet valve 172 of the two-position change-over type and a resistance pipe 173 are arranged also between the gas storage container 164 and the vacuum pump 168. The resistance pipe 173 is provided with a bypass line which has a bypass valve 174.
With this device, when the gas is introduced, the valves 165, 171 are opened, the valves 172, 163 are closed, and the state of pressure is monitored at all times by the pressure sensor 169. Upon the gas pressure exceeding a set value, the valve 171 is closed and the valve 163 is opened. When the gas is discharged, the valves 163, 171 are closed, the valves 172, 174 are opened, and the state of pressure is always monitored by the pressure sensor 169 as in the case of introduction of the gas. Upon the gas pressure dropping below a set value, the valve 172 is closed, and the valve 163 is opened. Thus, feedback control is resorted to for closing the valves 171, 172 after the set pressure is reached and therefore requires pressure monitoring means of high time resolution or a high-speed data logger. Delay in response leads to a great deviation from the set value.
Since the manual valve 165 is always left open during the introduction and discharge of the gas, the valve is free of the objectionable wear due to repeated opening and closing unlike the device of FIG. 20, but the flow rate of the gas is adjusted by varying the opening degree of valve and is accordingly difficult to adjust finely.
An object of the present invention which has overcome the foregoing problems is to provide a measuring apparatus and a measuring method which readily permit fine adjustment of set pressure of the gas to be supplied or discharged without necessitating a high-speed data logger or the like.